1. Field of the Invention
The present invention relates generally to communication systems utilizing a frequency shift keyed technique and more particularly, to an improved detection system using a tri-state phase detector.
2. Description of the Prior Art
Many communication systems including those which utilize existing power lines use digital modulation techniques in signal transmission. One method of digital transmission is phase shift keyed (PSK) transmission. In PSK systems, the phase of the carrier wave is shifted by 180 degrees to indicate in binary a "mark" or "1" and transmitted without shifting to indicate a space or a "0". Another digital transmission technique which is becoming more important involves frequency shift keyed (FSK) modulation. This is the type employed in the present invention. In this type of a system, one frequency is used to indicate a "mark" or "1" and another distinct frequency to indicate a space or "0". Frequency shift keying involves the modulation of the base or carrier frequency to shift that frequency by predetermined increments in response to particular data to be transmitted.
In a frequency shift keying system, the frequency shift phase is continuous, i.e., the transmitted signal is a sinusoidal signal which varies in frequency but has no time phase shift continuity. Normally, a binary "1" or a "mark" signal is transmitted at a frequency above a selected center or carrier center frequency or "carrier Plus" frequency and a space or binary "0" is transmitted at a frequency below the center frequency of the carrier or "carrier minus" frequency. In such systems the differential between a transmitted, and thus received frequency, and the center frequency of the carrier may be made equal to or greater than the modulation rate, also known as the data rate, or bit rate, divided by two.
Systems for receiving frequency shift keyed transmission signals are well-known. Such systems must discriminate accurately between the carrier minus and carrier plus frequencies and reject spurious or "noise signals" which also may be transmitted. Many techniques and schemes have been used in the prior art in an attempt to achieve better, more accurate signal discrimination. These include the use of various input logic schemes in conjunction with bandpass filters, or the like to check for the presence or absence of certain frequencies.
In the prior art, in order to achieve commercially feasible reliability, most discriminator schemes have had to be quite complicated. There has long been a need for simplification of these systems so that inexpensive reliable units would be available. It would be desirable to employ a system using a tri-state phase detector in conjunction with an integrating circuit, the output of which would be used as the input to a comparator which, in turn, would be utilized to activate a latch or other signal utilization means. In the prior art, however, it has been difficult, if not impossible, to provide reliable FSK discrimination using a tri-state detector in combination with an integrating circuit because of the lack of phase information between the output of the tri-state and the bit time. Thus, if the phase relationship is unknown, at least part of the information fed to the integrating circuit can be wrong and this, of course, can lead to a misinterpretation of the input data signal.